Talking to the UNIX build system: config.m4

The config.m4 file for an extension tells the UNIX
build system what configure options your extension
supports, what external libraries and includes you require, and what source
files are to be compiled as part of it. A reference to all the commonly used
autoconf macros, both PHP-specific and those built into autoconf, is given
in the Zend Engine 2 API reference section.

Astuce

The version of autoconf you have installed makes a difference
when developing an extension. For PHP 5.3 and earlier, you will have the best results
with autoconf version 2.13 but versions up to and including 2.59
will work. For PHP 5.4 and later the oldest autoconf version you can
use is 2.59 and you will have better results with later versions.

A short introduction to autoconf syntax

config.m4 files are written using the GNU
autoconf syntax. It can be described in a nutshell as
shell scripting augmented by a powerful macro language. Comments are
delimited by the string dnl, and strings are quoted
using left and right brackets (e.g. [ and
]). Quoting of strings can be nested as many times as
needed. A full reference to the syntax can be found in the
autoconf manual at
» http://www.gnu.org/software/autoconf/manual/.

PHP_ARG_*: Giving users the option

The very first thing seen in the example config.m4
above, aside from a couple of comments, are three lines using
PHP_ARG_WITH() and PHP_ARG_ENABLE().
These provide configure with the options and help text
seen when running ./configure --help. As the names
suggest, the difference between the two is whether they create a
--with-*
option or an
--enable-*
option. Every extension should
provide at least one or the other with the extension name, so that users
can choose whether or not to build the extension into PHP. By convention,
PHP_ARG_WITH() is used for an option which takes a
parameter, such as the location of a library or program required by an
extension, while PHP_ARG_ENABLE() is used for an option
which represents a simple flag.

Regardless of the order in which options are specified on the command line
when configure is called, the checks will be run in the
order they are specified in config.m4.

Processing the user's choices

Now that config.m4 can provide the user with some
choices of what to do, it's time to act upon those choices. In the example
above, the obvious default for all three options, if any of them are
unspecified, is "no". As a matter of convention, it is best to
use this as the default for the option which enables the extension, as it
will be overridden by phpize for extensions built
separately, and should not clutter the extension space by default when
being built into PHP. The code to process the three options is by far the
most complicated.

Handling the --with-example[=FILE] option

The first check made of the
--with-example[=FILE]
option is whether
it was set at all. As this option controls the inclusion of the entire
extension, if it was unspecified, given in the negative form
(--without-example
), or given the value
"no", nothing else is done at all. In the example above, it is
specified with the value
/some/library/path/example-config, so the first test
succeeds.

Next, the code calls AC_MSG_CHECKING(), an
autoconf macro which outputs a standard
"checking for something" line, and checks whether the user gave
an explicit path to the fictional example-config. In
this example, PHP_EXAMPLE got the value
/some/library/path/example-config, which is now copied
into the EXAMPLE_PATH variable. Had the user specified only
--with-example
, the code would have
executed $php_shtool path $EXAMPLE_CONFIG, which would
try to guess the location of example-config using the
user's current PATH. Either way, the next step is to
check whether the chosen EXAMPLE_PATH is a regular
file, is executable, and can be run successfully. If so,
AC_MSG_RESULT() is called, which completes the output
line started by AC_MSG_CHECKING(). Otherwise,
AC_MSG_ERROR() is called, which prints the given
message and halts configure immediately.

The code now determines some site-specific configuration information by
running example-config several times. The next call is
to PHP_CHECK_LIBRARY(), a macro provided by the PHP
buildsystem as a wrapper around autoconf's
AC_CHECK_LIB(). PHP_CHECK_LIBRARY()
attempts to compile, link, and run a program which calls the symbol
specified by the second parameter in the library specified by the first,
using the string given in the fifth as extra linker options. If the
attempt succeeds, the script given in the third parameter is run. This
script tells the PHP buildsystem to extract include paths, library paths,
and library names from the raw option strings
example-config provided. If the attempt fails, the
script in the fourth parameter is run instead. In this case,
AC_MSG_ERROR() is called to stop processing.

Handling the --enable-example-debug option

Processing the --enable-example-debug
is
much simpler. A simple check for its truth value is performed. If that
check succeeds, AC_DEFINE() is called to make the C
macro USE_EXAMPLE_DEBUG available to the source of the
extension. The third parameter is a comment string for
config.h; it is safe to leave this empty, and often is.

Handling the --with-example-extra=DIR option

For the sake of this example, the fictional "extra"
functionality requested by the
--with-example-extra=DIR
option does not
share the fictional example-config program, nor does it
have any default paths to search. Therefore, the user is required to
provide the installation prefix of the necessary library. This setup is
somewhat unlikely in a real-world extension, but is considered
illustrative.

The code begins in a now-familiar way by checking the truth value of
PHP_EXAMPLE_EXTRA. If a negative form was provided, no
further processing is done; the user did not request extra functionality.
If a positive form was provided without a parameter,
AC_MSG_ERROR() is called to halt processing. The next
step is another invocation of PHP_CHECK_LIBRARY(). This
time, since there is no set of predefined compiler options provided,
PHP_ADD_INCLUDE() and
PHP_ADD_LIBRARY_WITH_PATH() are used to construct the
necessary include paths, library paths, and library flags for the extra
functionality. AC_DEFINE() is also called to indicate
to the code that the extra functionality was both requested and available,
and a variable is set to tell later code that there are extra source files
to build. If the check fails, the familiar
AC_MSG_ERROR() is called. A different way to handle the
failure would have been to call AC_MSG_WARNING()
instead, e.g.:

AC_MSG_WARNING([example-extra lib not found. example will be built without extra functionality.])

In this case, configure would print a warning message
rather than an error, and continue processing. Which way such failures are
handled is a design decision left to the extension developer.

Telling the buildsystem what was decided

With all the necessary includes and libraries specified, with all the
options processed and macros defined, one more thing remains to be done:
The build system must be told to build the extension itself, and which
files are to be used for that. To do this, the
PHP_NEW_EXTENSION() macro is called. The first parameter
is the name of the extension, which is the same as the name of the
directory containing it. The second parameter is the list of all source
files which are part of the extension. See
PHP_ADD_BUILD_DIR() for information about adding source
files in subdirectories to the build process. The third parameter should
always be $ext_shared, a value which was determined by
configure when PHP_ARG_WITH() was
called for --with-example[=FILE]
. The
fourth parameter specifies a "SAPI class", and is only useful for
extensions which require the CGI or CLI SAPIs specifically. It should be
left empty in all other cases. The fifth parameter specifies a list of
flags to be added to CFLAGS while building the
extension; the sixth is a boolean value which, if "yes", will
force the entire extension to be built using $CXX
instead of $CC. All parameters after the third are
optional. Finally, PHP_SUBST() is called to enable
shared builds of the extension. See Extension FAQs for
more information on disabling support for building an extension in shared
mode.

The counter extension's config.m4 file

The counter extension previously documented has a much simpler
config.m4 file than that described above, as it doesn't
make use of many buildsystem features. This is a preferred method of
operation for any extension that doesn't use an external or bundled library.

Exemple #3 counter's config.m4 file

dnl

$

Id$
dnl config.m4 for extension counter
PHP_ARG_ENABLE(counter, for counter support,
[ --enable-counter Include counter support])
dnl Check whether the extension is enabled at all
if test "$PHP_COUNTER" != "no"; then
dnl Finally, tell the build system about the extension and what files are needed
PHP_NEW_EXTENSION(counter, counter.c counter_util.c, $ext_shared)
PHP_SUBST(COUNTER_SHARED_LIBADD)
fi